Separation membranes are in use in many fields, such as gas separation, pervaporation, ultrafiltration, reverse osmosis, dialysis and electrodialysis.
The optimum separation membrane combines high selectivity with high flux. Thus the membrane industry has engaged in an ongoing quest for membranes with improved flux/selectivity performance. Gas separation is a particularly active research area.
In recent years, some glassy polymer materials with extremely high gas permeabilities have been synthesized and formed into membranes. The best known and most studied of these is poly(1-trimethylsilyl-1-propyne) [PTMSP], a substituted polyacetylene.
PTMSP has been found to exhibit anomalous properties for a glassy material, in that PTMSP membranes are more permeable to larger, more condensable molecules than to smaller, less condensable molecules. Also, they have higher mixed gas selectivity than pure gas selectivity for at least some gas pairs. These properties were made use of in U.S. Pat. No. 5,281,255, for example. PTMSP has become known as a "superglassy" material, in part because of the unusual combination of permeability and selectivity properties.
Other polyacetylenes are known that exhibit superglassy properties similar to those of PTMSP. For example, U.S. Pat. No. 5,707,423 describes hydrocarbon-based polymers that exhibit selectivity in favor of condensable organic molecules over smaller, less condensable molecules, and in which measured mixed gas selectivity is higher than calculated pure gas selectivity.
It is known to incorporate various types of small particles, generally called fillers, into polymeric membranes to modify the membrane properties. U.S. Pat. No. 4,093,515 discloses a membrane using multiple layers of silicone rubber polymer, in which one layer contains a silica filler and another contains a carbon black filler. The membrane is used in an artificial lung for blood oxygenation during surgery.
U.S. Pat. No. 4,279,752 discloses a poly(vinyl alcohol) membrane prepared from a solution containing colloidal silica particles or silica powder. In this case, most of the silica is subsequently leached out to form a microporous membrane.
U.S. Pat. No. 4,925,562 discloses silicone rubber membranes filled with zeolite particles. The membrane is used to separate alcohols from water, for example.
U.S. Pat. No. 5,173,189 describes the incorporation of unspecified particles into a support for a membrane used for organic solvent/water separations.
U.S. Pat. No. 5,755,967 also concerns a silicone rubber membrane filled with zeolite. In this case the membrane is used to separate butanol and other products from fermentation broth.
A chapter by J. H. Petropoulos entitled "Mechanism and Theories for Sorption and Diffusion of Gases in Polymers" in Polymeric Gas Separation Membranes, D. R. Paul and Yu. P. Yampol'skii (eds.), CRC Press, 1994, presents equations and graphs indicating the effect on permeability of the presence of a less-permeable phase dispersed in a more-permeable phase.
A chapter by R. M. Barrer entitled "Diffusion and Permeation in Heterogeneous Media" in Diffusion in Polymers, J. Crank and G. S. Park (eds.), Academic Press, 1968, presents several theoretical treatments and experimental data showing the decrease in permeability that occurs when impermeable particles are dispersed in a permeable polymer.
A paper by A. Jonquieres and A. Fane entitled "Filled and unfilled composite GFT PDMS membranes for the recovery of butanols from dilute aqueous solutions: influence of alcohol parity", in Journal of Membrane Science, Vol. 125, pages 245-255 (1997) discusses the use of filled silicone rubber membranes for butanol/water separations. The filled membranes provide lower fluxes but higher selectivity than their unfilled membranes.
A paper by S. Kumar et al. entitled "Permeation in filled membranes: Role of solute-filler interactions" in Journal of Membrane Science, Vol. 134, pages 225-233 (1997) compares theoretical and experimental results for pervaporation of alcohols and carboxylic acids through filled membranes containing adsorptive fillers. The paper points out that filled membranes, because of their reduced permeability, can be used for as barrier materials in packaging.
A paper by J. P. Boom et al. entitled "Transport through zeolite filled polymeric membranes", in Journal of Membrane Science, Vol. 138, pages 237-258 (1998) includes experimental results and numerical modeling studies of separation of methanol/toluene mixtures by means of rubbery membranes containing hydrophilic and hydrophobic zeolites.
A paper by S. Ulutan and T. Nakagawa entitled "Separability of ethanol and water mixtures through PTMSP-silica membranes in pervaporation", in Journal of Membrane Science, Vol. 143, pages 275-284 (1998) discusses ethanol/water separation pervaporation experiments performed with membranes in which large silica particles with average diameters of 5 .mu.m were loaded asymmetrically into a PTMSP membrane. The loaded membranes had poorer separation factors than the pure PTMSP membranes.
A paper by M. Moaddeb and W. J. Koros entitled "Effect of colloidal silica incorporation on oxygen/nitrogen separation properties of ceramic-supported 6FDA-IPDA thin films" in Journal of Membrane Science, Vol. 111, pages 283-290 (1996) discusses a membrane formed on a ceramic support. The support material is coated with silica particles, which impregnate the pores of the support surface. The structure is then coated with a polyimide solution. The resulting membrane exhibits improved properties for oxygen/nitrogen separation.
Another paper by Moaddeb and Koros entitled "Gas transport properties of thin polymeric membranes in the presence of silicon dioxide particles" in Journal of Membrane Science, Vol. 125, pages 143-163 (1997) provides additional experimental data and discussion compared with the earlier paper. The polymers used are conventional glassy materials with low free volumes ranging from 0.16 to 0.19. The paper reports an increase in oxygen/nitrogen selectivity and an apparent increase in oxygen permeability compared with films of polymer alone.